What is Blow Molding?
Blow molding is a manufacturing process transforming thermoplastics into usable, hollow parts with uniform wall thicknesses, such as a water bottle. Some thermoplastic used in blow molding include but are not limited to HDPE, LDPE, PP, PVC, PET, PC, etc.
What is the general blow molding process?
Unlike injection molding, where molten plastic is forced or injected into a mold cavity, the blow molding process involves molten plastic being blown against the mold cavity walls (kind of like a balloon) by pressurized air. The resulting product has a hollow center in the shape of the mold.
While the general blow molding process is the same, it can be divided into three manufacturing types:
- Extrusion blow moldingThe simplest and most versatile of the three, extrusion blow molding allows for a wide variety of container shapes, sizes and neck openings. It also allows for the production of bottles with handles.Although extrusion blow molding offers a high rate of production with low tooling cost, there is a high scrap rate with limited control over wall thickness.
- Injection blow moldingThe least used type of blow molding, injection blow molding is used mainly for smaller, precise products with close tolerance threaded necks, wide mouth openings, solid handles, and or highly styled shapes. The finished product has good overall distribution and generally do not need trimming or reaming.Compared to extrusion blow molding, injection blow molding has higher production rates with low scrap percentages. Due to the need of producing two sets of molds (compared to one set needed for extrusion blow molding), injection blow molding has higher setup costs and limited flexibility on product gram weight.
- Stretch blow moldingBest known for producing PET bottles (water, juice, soda, etc.), stretch blow molding has been around since the 1970s.Its major advantage is the ability to stretch the preform in both the hoop and the axial direction. This biaxial stretching increases tensile strength, barrier properties, drop impact, clarity, and top load in the final product. This also allows the final product to be 10-15% lower in weight (less material needed).In general, stretch blow molding is preferred over injection blow molding as it is more accurate offering better overall quality.